When preparing and completing an underground well, for example of the kind used in the recovery of petroleum, strict safety requirements are imposed on those in charge. It is important to protect personnel, the surrounding environment and the drilling and production equipment from the formations being operated in. Large pressure and temperature differences may make the work on the well challenging. Well barriers are loops with one or more cooperating well-barrier elements which prevent fluids from undesirably flowing from the formation, either into another formation or up to the surface. According to the NORSOK standard D-010 (“Well integrity in drilling and well operations”), one barrier loop is required in a well in which there is a risk of uncontrolled cross flows between the formation zones of the well, whereas two barrier loops are required in wells in which there is a risk of uncontrolled blowouts from the well into the surroundings. Intact barrier loops are a condition for the work to be carried out in accordance with the standard and with no or minimal risk of undesired events. Various forms of plugs may be important well-barrier elements in both primary and secondary barrier loops. In practice, two spaced-apart plugs are often established in the well. It is desirable, and in several countries required by law, to monitor the integrity of both plugs. In practice, only the pressure above the upper plug is monitored, so that an undesired pressure increase in the space between the two plugs, owing to a failure of the lower plug, will not be picked up. So, a barrier element may have been lost without this being shown on the surface. Often, barrier plugs will be installed because work above the upper plug in the well has been planned. Getting components installed that can monitor the pressure between the two barrier plugs may therefore be challenging. Using equipment, for example cable glands, that may affect the functionality of the upper barrier plug is not desirable either.